This is the 620th most frequent Slovenian word.
mnogi
"Many."
In this sentence, 'mnogi' acts as a pronoun representing people in general, indicating that numerous individuals held that opinion.
Mnogi so menili, da je ideja zanimiva.
Many thought that the idea was interesting.
Here, 'mnogi' is used as a determiner meaning 'many' to describe a quantity of people.
Mnogi ljudje radi potujejo.
Many people love to travel.
Here, 'mnogi' is inflected as 'mnoge' and functions as an adjective modifying 'books', giving them the quantitative attribute of being 'many'.